Quantum state engineering by periodical two-step modulation in an atomic system.

By periodical two-step modulation, we demonstrate that the dynamics of a multilevel system can evolve even in a multiple large detunings regime and provide the effective Hamiltonian (of interest) for this system. We then illustrate this periodical modulation in quantum state engineering, including achieving direct transition from the ground state to the Rydberg state or the desired superposition of two Rydberg states without satisfying the two-photon resonance condition, switching between the Rydberg blockade regime and the Rydberg antiblockade regime, stimulating distinct atomic transitions by the same laser field, and implementing selective transitions in the same multilevel system. Particularly, it is robust against perturbation of control parameters. Another advantage is that the waveform of the laser field has a simple square-wave form, which is readily implemented in experiments. Thus, it offers us a novel method of quantum state engineering in quantum information processing.